1. Field of the Invention
The present invention relates to a device and method for directly injecting a test signal into a cable, so that the isolating characteristics of the system immunity response can be evaluated.
2. Background Information
Computer systems typically include a number of electrical components, such as printed circuit boards, that are electrically coupled together. One way to accomplish this is to use electrical cables to electrically join the respective electrical components. For example, the electrical cables allow electrical signals, via signal wires, to be transmitted from one printed circuit board to another printed circuit board, so that various electronic devices on the one printed circuit board can communicate with electronic devices on the other printed circuit board. Cables are also used to supply power to and from the various electrical components.
However, many electrical devices, both inside of the computer system and external to the computer system, when operated, generate emissions that include electromagnetic radiation. This electromagnetic radiation may travel through the air, and be received by the signal wires of the cable. The received electromagnetic radiation may then adversely affect the operation of the electronic devices to which the cable is connected, causing the computer system to malfunction. When this electromagnetic radiation influences the proper functioning of the electronic devices, the result is known as electromagnetic interference (also known as EMI). Thus, in order to ensure reliable operation, the signal wires within the cables must be shielded against outside interference. For example, it is known to wrap or encircle all of the wires in a cable by a conducting shield, usually foil or braided wire, which is connected to an external jacket, such as a metal housing shield of a plug, at each end of the cable. The metal housing shield is then coupled to a ground potential, so that any electromagnetic radiation is conducted to the ground potential, thereby preventing the radiation from adversely affecting the electronic devices coupled to the cable.
Certain regulations have been established which provide a frequency range that a shield must be capable of isolating the cable""s signal wires from, to ensure that the conducting shield of the cable provides adequate protection against electromagnetic interference. For example, the International Electrotechnical Commission (IEC) has established and published various standards that stipulate the frequency ranges that the shields must provide isolation for. One published standard (IEC Standard 61000-4-6, entitled Immunity to Conducted Disturbances Induced by Radio Frequency Fields) provides that a shield must isolate the internal cable wires from electromagnetic interference radiation in the frequency range between 150 kilohertz to 80 megahertz. Moreover, this standard provides criteria that must be in place when the testing is performed. For example, this standard dictates that in testing a system""s immunity response, a test signal (which simulates electromagnetic interference radiation in the noted frequency range) must be directly injected into the shield of the cable, via a 100-Ohm resister, while the cable is disposed 30 mm from the floor, in order to test the system immunity response. If the system is not isolated from the test signal, for example if the shield does not isolate the internal cable wires from the test signal, then the system will be deemed as being unsatisfactory for use in certain environments. For example, all systems marketed in Europe must meet the requirements of the standard.
However, although the above standard provides guidelines for testing the system, the standard is silent on how to actually perform the test. Thus, it is up to the end user of the system, to provide a mechanism and procedure that will allow the system to be tested to ensure that they meet the shielding requirements of this and other various standards.
Moreover, it is known to test a system for susceptibility to electromagnetic radiation interference by injecting the test signal using antennas. However, a test signal injected using an antenna is not directly injected, but instead is remotely injected, and thus would not meet the requirements of a standard that requires a direct injection of a test signal. Thus, there is a need for a device that allows a test signal to be directly injected into a shield of a cable. Moreover, there is a need for a procedure for implementing the direct injecting of a test signal into a shield of a cable.
Further, it is desirable to perform the direct injecting of the test signal into the shield without damaging the cable. That is, typically the shield is covered with an outer insulating sheathing (cover). Obviously, to access the shield for the direct injection, the cover must be penetrated in some manner. However, it would be desirable if this penetration did not damage the cable to an extent that the cable could no longer be used. Thus, there is a need for a device that allows for the direct injecting of a test signal into a shield of a cable, that does not damage the cable.
Furthermore, as previously discussed, the shield surrounds the wires of the cable. However, for the testing to be accurately performed, it is important that the device used for injecting the test signal does not contact the underlying wires. If the device should contact one or more of the wires, then the wires will conduct the test signal, skewing the results of any test. Thus, there is a need for a device that allows for the direct injecting of a test signal into a shield of a cable, while ensuring that the wires within the cable are not contacted by the device.
It is, therefore, a principal object of this invention to provide a device and method for directly injecting a test signal into a cable.
It is another object of the invention to provide a device and method for directly injecting a test signal into a cable that solves the above-mentioned problems.
These and other objects of the present invention are accomplished by the device and method for directly injecting a test signal into a cable disclosed herein.
According to one aspect of the invention, a device for directly injecting a test signal into a cable is provided. The device includes a lower clamp member, and an upper clamp member superposed over the lower clamp member. Both the lower clamp member and the upper clamp member are preferably formed from an insulating material, such as plastic. Forming these features of an insulating material ensures that these components will not form a conductive path for the injected test signal.
In an exemplary aspect of the invention, the device has a conductive piercing member, such as a metal pin, disposed on the lower clamp member to project toward the upper clamp member. In use, the conductive piercing member will pierce an outer sheathing (which is typically an insulator) of the cable, and engage an inner conductive shield of the cable, in a manner that will be subsequently described. Moreover, the conductive piercing member preferably has a sharp, pointed tip that can easily pierce the outer sheathing of the cable.
The device further has a conductive coupling member disposed on an upper surface of the lower clamp member. The conductive coupling member is electrically coupled to the conductive piercing member.
In the exemplary embodiment, the conductive piercing member is integral to the conductive coupling member. For example, the conductive piercing member can be a pin that is welded or soldered, or otherwise permanently fastened to the conductive coupling member. Alternatively, the conductive piercing member can be stamped from the conductive coupling member. With this arrangement, the conductive piercing member would have an inverted V-shaped configuration to facilitate the stamping of a suitably sharp piercing member.
The device may further include a coaxial connector that is electrically coupled, via a resistor for example, to the conductive coupling member. The coaxial connector facilitates the connection of a signal generator to the device. For example, one end of a standard coaxial cable can be easily connected to the coaxial connector, and another end of the standard coaxial cable can be connected to the signal generator.
The conductive coupling member may have an L-shaped configuration, with a short leg of the L projecting over a side of the lower clamp member. This short leg can then be fastened, for example using a screw, to the resistor.
The lower clamp member may be provided with a recess for accommodating the short leg of the conductive coupling member, the end of the coaxial connector, and the resistor. The recess advantageously allows these features to be somewhat protected by the lower clamp member, and helps to reduce the size of the device.
The upper clamp member may be provided with a groove disposed on a lower surface, and across a width, thereof. The groove is arranged to be essentially parallel to the conductive coupling member and positioned directly thereover. The groove helps to position the cable to be tested in the proper orientation relative to the conductive piercing member and helps to ensure that the cable will not inadvertently shift when the upper clamp member and the lower clamp member are clamped together.
To facilitate the clamping of the upper clamp member to the lower clamp member, the device further includes a knob having a threaded shaft. The knob is disposed on the upper surface of the upper clamp member. The threaded shaft projects freely through the upper clamp member and is threadably engaged with a female threaded hole formed in the upper surface of the lower clamp member. As the knob is turned in a clockwise direction, for example, the threaded shaft engages with the female threaded hole to pull the upper clamp member toward the lower clamp member. This arrangement advantageously allows the upper and lower clamp members to be moved toward each other in precise incremental amounts.
In the exemplary embodiment, a rear of the upper clamp member is hinged to the rear of the lower clamp member. This hinge ensures that the groove (and thus the cable to be tested) retains its position relative to the conductive piercing member. For example, the device can include a small, relatively thin, flat piece of spring steel that is attached, for example using screws, to the rear faces of the upper clamp member and the lower clamp member. This arrangement is advantageous in that the spring steel, once attached, will prevent the upper clamp member from inadvertently shifting laterally and/or longitudinally relative to the lower clamp member. Moreover, the spring steel may exert a force that naturally urges the upper clamp member away from the lower clamp member. However, it is also contemplated that instead of spring steel, the upper clamp member can be hinged to the lower clamp member using any conventional hinge arrangement without departing from the spirit and scope of the invention.
The device may further include a relatively flat metal plate upon which the upper and lower clamp members are disposed. The metal plate can include spaced-apart, upwardly projecting walls, which define a space that the lower clamp member can be disposed within. One of the walls may have the coaxial connector attached thereto. For example, an external jacket of the coaxial connector can be coupled to the wall. Further, the flat metal plate may be coupled to earth around.
To test the isolation capability of the system, the ends of the cable are first connected to respective electrical devices. As is conventional, the shield of the cable will thus be connected to earth ground via its connection to the electrical devices. Thereafter, a continuity tester may be coupled to the coaxial connector to test for continuity between the external jacket and the inner connector of the coaxial connector. As will be appreciated, since the external jacket and the inner connector of the coaxial connector are electrically isolated from each other, at this stage there will be no continuity between these features.
Next, the user places the cable over the conductive piercing member, and under and within the groove formed in the upper clamp member. Thereafter, the user may slowly turn the knob in the clockwise direction for example, to thread the threaded shaft into the female threaded hole. This will cause the conductive piercing member to pierce the outer sheathing of the cable and subsequently come into contact with the inner shield. As soon as the conductive piercing member contacts the inner shield, a closed-circuit will be formed (due to the attachment of the continuity tester) between the inner shield, which is connected to earth ground via the devices to which the cable is connected, the metal plate which is likewise connected to earth ground, the conductive piercing member which is now electrically coupled to the inner shield, and to the coaxial connector which is electrically coupled to both the conductive piercing member and the metal plate. At this point, as soon as continuity is detected, the user stops turning the knob. Thus, the conductive piercing member is prevented from piercing any of the wires contained within the cable that is to be tested.
Next, the user removes the continuity tester and attaches the coaxial connector to the signal generator using a further cable. Thereafter, the signal generator is operated to generate a test signal, which is directly injected into the shield. The devices to which the tested cable is attached are then operated in their normal manner. These devices can then be monitored for any malfunctions. This monitoring can be accomplished in any conventional manner. If no malfunctions are detected, the system immunity response is deemed to have met the requirements of the standard. The device can thus be removed, and the cable can continue to be used for its intended purpose.